1. Field of the Invention
The invention relates generally to the field of communications. More particularly, the invention relates to counter-propagation of a first signal and a second signal on an optical conductor. A specific embodiment of the invention relates to simultaneous transport of analog video signals and Ethernet data.
2. Discussion of the Related Art
Network service providers, such as CATV (cable access television) system operators, who deliver analog video service over their optical transport networks may also desire to offer high-speed data services over the same existing optical networks. In order to minimize the number of optical fibers required for the transport of both analog video and high-speed data, wavelength division multiplexing (WDM) methods may be used to combine the optical carriers transporting such services onto a small number of optical fibers.
The simplest arrangement to implement simultaneous optical transport of video and high-speed data is to combine all downstream optical carriers from the central office, head-end or hub, both analog video and high-speed data, onto a single fiber, and then demultiplex them in a secondary hub or in the field as required to deliver service to subscribers. Upstream or return signals, both analog and high-speed data, would be multiplexed onto a single fiber and transported to the head-end or central office where they would be demultiplexed and processed.
Because of nonlinear optical interactions in the optical fiber used for signal transport, such an arrangement can lead to unacceptable levels of interference of the high-speed data signals with the analog video signals, which typically have very demanding requirements for signal-to-noise and distortion levels. As a specific example, the simultaneous transport of analog video signals using multiple radio frequency (RF) sub-carriers on a 1310 nm optical carrier co-propagating in the same fiber with longer wavelength optical carriers transporting standard Fast Ethernet (100 Mbps) can, under certain circumstances, lead to excessive levels of interference at 62.5 MHz, resulting in unacceptable degradation of the video signal on NTSC channel 3. This interference because of nonlinear optical interactions in the optical fiber used for signal transport is due to the co-propagation of the optical carriers transporting digital data and analog video.